The present invention relates to a preamplifier mainly for an optical communication system, disposed at the front end of an optical signal receiver circuit for converting a current signal from a photodiode to a voltage signal.
In an optical communication system, a preamplifier used in an optical signal receiver section has a function of converting a current signal supplied from a photodiode to a voltage signal. The current signal originates from an optical signal transmitted through an optical fiber. In general, a transimpedance amplifier is used as such a preamplifier.
A transimpedance amplifier is composed of an inverting amplifier and a feedback resistor Rf connected in parallel with the inverting amplifier between the input and output thereof. The 3 dB attenuation band fxe2x88x923 dB of transimpedance gain of a transimpedance amplifier is represented by:
fxe2x88x923 dB=(1+A)/2(Rfxc3x97Cin)
where A denotes the open gain of the inverting amplifier, Rf denotes the resistance of the feedback resistor, and Cin denotes the parasitic input capacitance at the input terminal of the inverting amplifier. From the above equation, it is found that to broaden the band of the transimpedance amplifier, the open gain A of the inverting amplifier must be increased.
Setting a high open gain A, however, fails to secure the band of the inverting amplifier itself. For this reason, it is quite difficult to attain a wide-band transimpedance amplifier with a single-stage inverting amplifier. Therefore, in general, an odd number of stages (normally, three stages) of low-gain, wide-band inverting amplifiers are put in cascade connection, to thereby obtain high gain and thus attain a wide-band transimpedance amplifier.
The conventional multistage configuration described above has a drawback that the phase largely rotates and thus a sufficient phase margin is not obtainable. To obtain stable characteristics, therefore, a phase compensation means of some type is required. One example of such a means is described in M. Nakamura et al, xe2x80x9cA 156-Mb/s CMOS Optical Receiver for Burst-Mode Transmissionxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 33, No. 8, pp. 1179-1187, August 1998, where three-stage inverting amplifiers are cascaded and feed-forward compensation is adopted for suppressing phase rotation.
In the conventional multistage configuration, three stages or more are generally required as described above. This not only increases power consumption, but also increases the chip area since passive elements such as capacitors and resistors are required for phase compensation.
An object of the present invention is providing a stable, low-power, wide-band transimpedance amplifier without the necessity of special phase compensation.
To attain the above object, a preamplifier according to the present invention has a configuration that the input of an inverting amplifier is connected to an input terminal, the output of the inverting amplifier is connected to the gate of a first transistor, the source of the first transistor and the source of a second transistor are connected to a first bias member, the drain of the first transistor and the gate of the second transistor are connected to a first load member, the drain of the second transistor is connected to a second load member and an output terminal, and a feedback resistor is connected between the input and output terminals.
An optical communication unit according to the present invention includes an optical receiver section for converting an optical signal transmitted through an optical fiber to a current signal by means of a photodiode to extract receiving data. The optical receiver section includes the preamplifier described above that converts the current signal to a voltage signal.
An optical detector according to the present invention includes the preamplifier described above. The optical detector is configured to convert a received optical signal to a current signal by means of a photodiode and then convert the current signal to a voltage signal by means of the preamiplifier.